1. Field of the Invention
This invention relates to a coating composition containing a chemically surface treated clay mixture, and methods of its use as a protective coating. The invention also relates to the surface treated clay mixture itself and a pigment mixture combining it and microspheres.
2. Description of the Related Art
Protective surface coatings are used in a wide variety of applications to provide a thin film barrier between the surface of a body needing protection and its immediate surrounding environment. Protective coatings of this sort have been used, for instance, on marine, aircraft, and industrial structures and parts. These protective coatings often are formulated to include a curable organic medium, pigments, and inorganic filler particles dispersed within the medium.
It is well known that the proper selection of the pigments has a profound influence on protective and other functional properties of protective coatings. The amount of organic medium present between the pigment particles, and some important aspects of the behavior of the medium are determined by the number, size, degree of dispersion and surface characteristics of the pigment particles. The pigments, as differentiated from the fillers provide many of the essential properties of the coating such as color, corrosion protection, durability and special rheological properties that address the practical aspects of coating application such as ease of application and firm build. Many conventional fillers used in coatings are commodities having lower cost than the base resin of the coating. For this reason, fillers are often used to reduce the cost of the coating. The amount of cost reduction that can be achieved through the use of commodity fillers is limited since their use at moderate levels may cause reduction of gloss and increase of viscosity. Ultimately, attempts to reduce cost by merely increasing the proportion of filler in the coating have been hampered by the drawback that a composition is reached beyond which there is insufficient medium to satisfactorily coat the filler particles and fill the interstices between them. A further increase in the pigment volume concentration (PVC) results in a sharp decline in mechanical and protective properties as a function of increased film porosity.
U.S. Pat. No. 4,171,228 describes a pigment composition for high build protective coatings material using a combination of two groups of pigments each of nodular (isometric) geometry comprised of relatively fine particles used in conjunction with smooth larger pigments. The large nodular pigments are exemplified as low density hollow or cellular microspheres made of glass, ceramic or polymer. The paints incorporating the combination of nodular pigments as prescribed in the ""228 patent are described as permitting an increase in the non-volatile content of the paints without viscosity increase. The ""228 patent represents that nodular pigments particles are necessary to achieve lower viscosity at high pigment loading scenarios, and to minimize pigment particle interlock and flow impedance.
Efforts in the past have been made to improve the properties and usefulness of solid inorganic filler and pigment particles by chemical surface treatment of them with organosilanes. U.S. Reissue Pat. No. RE30,450 discloses a finely divided particulate inorganic pigment, such as kaolin clay or hydrated silica, which is surface modified with 1-3% amino organosilane by spray drying a pigment slurry in which the silane is dispersed. The ""450 reissued patent discloses that the resulting modified pigment is useful as a component in thermosetting resins, such as polyurethanes, epoxy polymers, and others mentioned, as well being useful as a component in paper, paints, varnishes, inks, and paper coating compounds. U.S. Reissue Pat. No. RE30,699 discloses polyurethane polymer compositions which are filled with a kaolin clay modified with from 1-3% of an amino organosilane, which is formed by spray drying a pigment slurry in which the silane is dispersed.
U.S. Pat. No. 6,197,105 discloses sulfur- and vinyl-functional silane treated kaolin clays useful as pigments or extenders in rubber compositions, in which the chemically surface treated clays are obtained by predispersing or emulsifying the silane in water by means of a surfactant before mixing it with the clay particles.
U.S. Pat. No. 4,075,153 discloses use of aminosilane or mercaptosilane treated kaolin clay in a two-component chromate-containing primer system. The ""153 patent more specifically describes the use of silane-treated clay in at least one of the two components of the primer system. The first component of the primer solution of the ""153 patent includes a liquid solution of a resinous polyepoxide and the second component includes a liquid solution of an epoxy curing agent carrying a plurality of amino hydrogen atoms, while at least one inorganic corrosion-resistant chromate pigment also is included in at least one of the two components. In the ""153 patent, the primer formulations are disclosed for use as a primer for bonding a paint topcoat upon a metallic surface, usually aluminum, in the aircraft industry. The use of chromates in coatings currently may be subject to regulatory considerations.
A need exists for enhanced performance with respect to resin-based protective coatings and filler or pigment additives for such coatings, which permit such coatings to be manufactured at competitive costs yet without sacrificing or compromising the needed Theological, inhibitive and barrier properties, and without requiring heavy metal additives such as chromates.
The present invention relates to coating compositions and a pigment used therein, in which the coating composition comprises a curable or hardenable resin system into which the pigment is dispersed. The pigment includes a first pigment mixture that comprises clay particles as first and second particle groups that have been surface treated with epoxy-functionalized organosilane or amino-functionalized organosilane, and a second pigment mixture that comprises clay particles as third and fourth particle groups that have been surface treated with amino-functionalized organosilane or epoxy-functionalized organosilane. The second particle group has a median particle size that is at least twice as large as the median particle size of the first group, and a similar size relationship is provided in the third and fourth particle groups. In a further aspect, the first and second particle groups, as well as the third and fourth particle groups, differ in size by at least an order of magnitude relative to each other.
In another aspect, the pigment further includes microspheres surface treated with organosilane that have a median size that is at least an order of magnitude larger than the second particle group.
In one preferred aspect, the resin system comprises a curable resin side and a curing agent side. The first pigment mixture is dispersed in either the curable resin side or the curing agent side, while the second pigment mixture is dispersed in the other side of the resin system. In a particular embodiment of this arrangement, the resin system in the coatable composition is a two-part epoxy coating system including a curable epoxy and an amine curing agent.
In an alternative aspect, curable or hardenable resins other than epoxies, such as polyurethanes, alkyds, and so forth, can be used as the resin system in the practice of this invention with or without curing agents as applicable.
It has been discovered that the combined use of clays having the different organosilane surface treatment chemistries, in conjunction with the different sizes, as set forth herein, yields an economical coating composition having improved performance. The coating compositions of this invention provide advantages such as lower cost, lower viscosity at higher volume solids and lower volatile organic content without compromising chemical resistance or barrier properties.
Among other things, it surprisingly has been observed that resin-based coating systems containing pigment according to this invention have significantly reduced viscosities as compared to comparative coating compositions formulated similarly but differing by using only one of the groups of clay particles. In one aspect of the invention, this rheological phenomenon is exploited such that high build protective coating compositions are provided having significantly higher pigment content than otherwise possible using only one of the groups of clay particles as pigment. This generally reduces material costs because more costly resin is effectively displaced in the coating composition formulation by the comparatively less costly pigment materials used according to this invention.
The ability to increase the proportion of pigment in the coating compositions in the practice of this invention is considered attributable at least in part to the particle packing benefits derived from the unique clay mixture used as a pigment. Namely, in one aspect of the invention, a pigment for use in the coating compositions is a dry, flowable clay mixture or blend including organosilane treated fine and coarse particle clays differing in median particle size sufficient to to yield particle packing benefits. In one aspect, the coarse clays generally are at least twice as large as, and, more particularly, at least an order of magnitude larger than, the fine clays in median particle size.
In one preferred aspect, the fine particle clays used in the clay mixture have platelet shapes. High pigment content high solids coating compositions, such as greater than 46% (56% of the dry coating film) by volume and 50% (58% of the dry coating film) by weight, have been successfully implemented using the clay mixtures including such platelet shaped particles as the group of fine particles used in the clay mixture, such as illustrated in the examples included herein. This is considered an advance because there has been conventional thinking that nodular or isometric shaped pigment particles were needed or important to achieve optimal particle packing benefits in coating compositions. In addition, the present invention also does not require use of microsphere fillers, although they are optional extenders, to achieve the desired Theological effects, although there may be benefits to their use in certain coating applications and the present invention also embraces embodiments in which they are included.
In one aspect, kaolin clay is a preferred type of siliceous clay for use in the practice of the invention. Kaolin clay does not cause significant increases in compositional viscosity, and other clays, which may be used are less preferred.
The coating compositions are ready-to-use formulations that can be applied and distributed over a portion of a surface or substrate to be coated by any convenient method and means. The coating compositions of the present invention can be readily formulated in flowable liquid form. These coatings can be applied to a surface by spraying, brushing, dipping, or any other suitable technique. Applications of the coating compositions include metal corrosion protection (e.g., marine, pipelines), waterproofing (e.g., fabrics, concrete), mechanical protection (e.g., optical surfaces, indoor flooring), and electrical insulation (e.g., wires). In one preferred aspect, the coating compositions are useful as protective coatings, such as surface coatings and linings applied to tanks, pipes, and the like. The present invention also encompasses such methods of application of the coating compositions described herein.
In another aspect, the invention also relates to clay mixtures comprised of either one or the combination of the above-mentioned first and second pigment mixtures.